1. Field of the Invention
The present invention relates to a drying technique suitable for drying a semiconductor wafer, and more particularly to improvement to prevent defective dryness.
2. Description of the Background Art
FIG. 27 is a front sectional view showing a structure of a drying apparatus according to the prior art which is the background of the present invention. A drying apparatus 151 is formed to dry a semiconductor wafer. The drying apparatus 151 comprises a processing vessel 171 having an upper end opened. A cooling coil 162 is attached to an inside of an upper portion of a side wall of the processing vessel 171 along the side wall of the processing vessel 171. The cooling coil 162 is formed of a silica tube, in which cooling water is caused to flow.
A heater 170 is provided just below a bottom portion of the processing vessel 171. Furthermore, a pan 166 is fixed to a position between the bottom portion and an upper open end in the processing vessel 171. A pipe 180 for drain is connected to a bottom portion of the pan 166.
When the drying apparatus 151 is used, an IPA (isopropyl alcohol) 167 is first poured into the processing vessel 171. A depth of the processing vessel 171 is regulated in such a manner that a liquid level does not reach the bottom portion of the pan 166. The cooling water is caused to flow in the cooling coil 162.
When the heater 170 is turned on, the IPA 167 is heated. As a result, the IPA 167 is vaporized so that an IPA vapor 165 is generated. The IPA vapor 165 is filled into the processing vessel 171. The IPA vapor 165 is cooled and condenses in the vicinity of the cooling coil 162. More specifically, the cooling coil 162 serves to prevent the IPA vapor 165 from leaking out of the processing vessel 171.
Accordingly, the IPA 167 is stored in a liquid storing section 169 provided in the vicinity of the bottom portion of the processing vessel 171, and the IPA vapor 165 is filled into a vapor filling section 168 from a top of the IPA 167 to the vicinity of the cooling coil 162. After the IPA vapor 165 is filled into the vapor filling section 168, a processing is started for a semiconductor wafer 163 to be processed. After a rinsing processing is completed, a lot of semiconductor wafers 163 and a cassette 164 carrying them are suspended from a holding arm 161 and are inserted into the vapor filling section 168 from above the processing vessel 171. The cassette 164 carrying the semiconductor wafers 163 is held by the holding arm 161 just above the pan 166 as shown in FIG. 27.
Consequently, the IPA vapor 165 filled in the vapor filling section 168 condenses and dissolves into waterdrops which stick to surfaces of the semiconductor wafers 163 and the cassette 164. As a result, the waterdrops are essentially changed into IPA droplets. The IPA droplets slip from the surfaces of the semiconductor wafers 163 and the cassette 164. Thus, the semiconductor wafers 163 and the cassette 164 which are wet with the waterdrops can be dried. The slipping IPA droplets are collected by the pan 166 and are then discharged to an outside through the pipe 180.
When the drying processing is completed, the cassette 164 is pulled up by the holding arm 161 and is taken out of the processing vessel 171. Then, the cassette 164 thus taken out is delivered to a next processing step. Thereafter, new (i.e. unprocessed) semiconductor wafers 163 and a new cassette 164 are put into the processing vessel 171. Thus, the semiconductor wafers 163 and the cassette 164 are dried repeatedly.
In the drying apparatus 151 according to the prior art, however, the semiconductor wafers 163 to be processed are exposed to the IPA vapor 165, and the IPA vapor 165 is caused to condense on the surfaces of the semiconductor wafers 163 so that the surfaces can be dried. Under conditions that an amount of condensation of the IPA vapor 165 is insufficient, the drying processing is not fully performed so that defective dryness is caused.
In particular, the IPA vapor 165 condenses in unnecessary portions such as a surface of an internal wall of the processing vessel 171 as well as the semiconductor wafers 163 acting as objects to be processed which have been put in. Therefore, the amount of condensation of the IPA vapor 165 is insufficient in some cases. Thus, yield of a semiconductor device manufactured in the semiconductor wafer 163 is deteriorated by the defective dryness of the semiconductor wafer 163.